1. The Field of the Invention
This invention relates generally to friction stir welding wherein heat for creating a weld is generated by plunging a rotating pin of a tool into a workpiece. More specifically, the present invention relates to a new tool that is used in a friction stir welding process that enables the present invention to weld materials that are not functionally weldable using state of the art friction stir welding processes and tools, said materials including ferrous alloys such as stainless steel, and higher melting point super alloys that contain only small amounts of or no ferrous materials at all.
2. Background of the Invention
Friction welding has been used in industry for years. It is a solid-state process that yields large economic benefits because it avoids many problems associated with rapid solidification of molten material that occurs in traditional fusion welding processes.
One example of friction welding occurs when the ends of two pipes are pressed together while one pipe is rigidly held in place, and the other is pressed against it and turned. As heat is generated by friction, the ends of the pipes become plasticized. By quickly stopping rotation of the pipes, the two pipes fuse together. Note that in this case, the frictional heating is caused by the relative motion of the two parts to be joined.
In contrast, FIG. 1 is a perspective view of a tool being used for friction stir butt welding that is characterized by a generally cylindrical tool 10 having a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, wherein the pin of the tool is plunged into the plasticized workpiece material. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. The pin 14 is plunged into the workpiece 16 at the joint line 18. The frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that is generally indistinguishable from the workpiece material 16.
The prior art is replete with friction stir welding patents that teach the benefits of using the technique to obtain welds that have beneficial characteristics over contemporary fusion welding processes. These benefits include low distortion in long welds, no fumes, no porosity, no splatter, and excellent mechanical properties regarding tensile strength. Furthermore, the process has the advantage of using a non-consumable tool, no need for filler wire, no need for gas shielding, and a tolerance for imperfect weld preparations such as the presence of oxide in the weld region. The process is especially useful for preventing significant heat damage or otherwise altering the properties of the original material being welded.
However, it has long been a desire of industry to be able to weld materials that are presently functionally unweldable for friction stir welding. Thus, while friction stir welding is a very advantageous technique for welding non-ferrous alloys such as aluminum, brass and bronze, there has been no tool that is capable of functionally welding materials having higher melting points. It should be understood that functionally weldable materials are those that are weldable using friction stir welding in more than nominal lengths, and without destroying the tool.
Unfortunately, fusion welding alters or damages the alloy at the weld, thereby compromising the weld as a result of the defects or adverse phases which form in the weld during the welding process. In some cases, the non-metallic reinforcement material which has been joined with the original workpiece material to create the alloy is depleted at the weld. The result is a weld that has properties and characteristics which are different from the unaltered areas of the original workpiece material.
Until now, it has been the nature of friction stir welding that using a conventional friction stir welding tool or probe is worn down significantly so as to prevent functional welding of materials such as MMCs, ferrous alloys, and superalloys. Most tools simply do not work at all in MMCs, ferrous alloys, and superalloys. If a conventional tool could begin friction stir welding, the wear would be so significant that a probe would be torn apart after only a short distance. For example, some alloys will cause wear on a probe such that it can no longer function after welding for a distance of only inches.
Unfortunately, it is generally the case that it is not possible to simply insert a new tool and begin the friction stir welding process where the previous probe failed. If the weld is not continuous and uninterrupted, it is useless because of mechanical weakness. Furthermore, a portion of the tool is typically left behind in the workpiece material, also contributing to the mechanical weakness.
Therefore, it would be an advantage over the prior art to provide a new tool for use with the friction stir welding process that enables longer continuous and uninterrupted welding runs (functional welding) of materials that will cause a conventional tool to fail after a short distance. It would also be an advantage over the prior art if the new tool made it possible to friction stir weld materials that were previously too difficult to weld with conventional friction stir welding tools. It would also be an advantage to provide a tool that would enable friction stir welding with conventional workpiece materials, while exhibiting improved wear characteristics for the tool.
A first class of materials that would be desirable to friction stir weld but are functionally unweldable with conventional tools are known as metal matrix composites (MMCs). An MMC is a material having a metal phase and a ceramic phase. Examples of the ceramic phase include silicon carbide and boron carbide. A common metal used in MMCs is aluminum.
MMCs have desirable stiffness and wear characteristics, but they also have a low fracture toughness, thereby limiting applications. A good example of a use for MMCs is in disk brake rotors on vehicles, where stiffness, strength and wear provide advantages over present materials, and where the more brittle nature is generally not an issue. The MMC makes the rotor lighter than cast-iron, and the ceramic phase such as silicon carbide enables greater wear resistance.
Other important applications for MMCs include, but should no be considered limited to, drive shafts, cylinder liners, engine connecting rods, aircraft landing gear, aircraft engine components, bicycle frames, golf clubs, radiation shielding components, satellites, and aeronautical structures.
A second class of materials that would be desirable to friction stir weld, and which have much broader industrial applications, are ferrous alloys. Ferrous alloys include steel and stainless steel. Possible applications are far-ranging, and include the shipbuilding, aerospace, railway, construction and transportation industries. The stainless steel market alone is at least five times greater than the market for aluminum alloys. It has been determined that steels and stainless steels represent more than 80% of welded products, making the ability to friction stir weld highly desirable.
Finally, a third class of materials that would be desirable to friction stir weld, have broad industrial applications, have a higher melting point than ferrous alloys, and either have a small amount of iron or none, are the super alloys. Superalloys are nickel-, iron-nickel, and cobalt-base alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is noted that titanium is also a desirable material to friction stir weld. Titanium is a non-ferrous material, but has a higher melting point than other non-ferrous materials.
There are significant challenges that have so far prevented the creation of a tool that can functionally weld MMCs, ferrous alloys, and superalloys. Some of these challenges only became apparent during experimentation as the inventors initially attempted to modify existing tools that can friction stir weld non-ferrous alloys. These challenges and the evolution of the tool will be discussed so as to enable the reader to practice the invention.
It is an object of the present invention to provide a new tool for use in friction stir welding that has improved wear characteristics over conventional tools.
It is another object to provide the new tool that includes a superabrasive material that enables friction stir welding of MMCs, ferrous alloys, and superalloys, as well as non-ferrous alloys.
It is another object to provide the new tool that enables improved weld characteristics for the non-ferrous alloys.
It is another object to provide the new tool that enables finishing costs of a welded material to be reduced.
It is another object to provide the new tool that has an improved geometry to reduce wear of the tool when friction stir welding MMCs, ferrous alloys and superalloys.
It is another object to reduce thermal, mechanical and chemical wear of the new tool.
It is another object to provide thermal management for the new tool.
In a preferred embodiment, the present invention is a tool for friction stir welding MMCs, ferrous alloys, non-ferrous alloys, and superalloys, the tool including a shank, a shoulder, and a pin disposed through the shoulder and into the shank, wherein the pin and the shoulder at least include a coating comprised of a superabrasive material, the pin and shoulder being designed to reduce stress risers, disposing a collar around a portion of the shoulder and the shank to thereby inhibit rotational movement of the shoulder relative to the shank, and incorporating thermal management by providing a thermal flow barrier between the shoulder and the shank, and between the collar and the tool.
In a first aspect of the invention, the shank, shoulder, and pin are separate components that are coupled together to form the friction stir welding tool, wherein the shoulder and the shank include a superabrasive coating.
In a second aspect of the invention, the shank and the shoulder are a monolithic element including a superabrasive coating over at least a portion thereof, and having a separate pin with a superabrasive coating.
In a third aspect of the invention, the shank, shoulder and pin are a monolithic element having a superabrasive coating covering at least a portion thereof.
In a fourth aspect of the invention, thermal management of heat using thermal flow barriers within the tool enables sufficient heat to be generated at the pin to enable friction stir welding, while protecting a tool holder from heat damage.
In a fifth aspect of the invention, stress risers are reduced on the pin, larger radii are provided on the shoulder, and pin diameter is increased to thereby enable friction stir welding of MMCs, ferrous alloys, and superalloys.
In a sixth aspect of the invention, the tool includes at least one CVD, ion-beam implanted, and/or PVD coating disposed over the superabrasive coating to thereby increase resistance to chemical and mechanical wear.
In a seventh aspect of the invention, the tool is coated with a whisker reinforced superabrasive in order to decrease spalling of the superabrasive coating.
In an eighth aspect of the invention, flats are disposed along the lengthwise axis of the tool to thereby prevent separation of the tool into component elements during translational motion of the tool.
In a ninth aspect of the invention, the superabrasive coating is selected based upon a desired balance between chemical wear and mechanical wear.
In a tenth aspect of the invention, the superabrasive coating is selected based upon the characteristic of having a low coefficient of friction that prevents the workpiece material from adhering to the tool, thereby reducing wear of the tool.
These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings.